I also wanted to make a equalizer, and i came up with this design (the ground here is the input ground (if that wasnt obvious)) dunno if this would work, but i think it should... maybe would mess up the sound really bad...

Not really I mean the way you arrange and connect all the grounds electrically because although they are essentially the same point, the resistance of the wire and PCB print means that electrically they are not. This is the number one cause of amps that are unstable and amps that have noise and hum and buzzes.

i knew that it wouldent be good in the beginning, but few moments later, (when i was going to sleep) i realized, that eaven if it would not have noises, then with lower hz tuners i would also adjust the higher pitches

later today i realized, that i can make an equalizer as another project and use the 5v of pc usb port as supply voltage for an active equalizer... but thats for another time

so i wanted to choose a heatsink for the chip... i know that the formula is
T(Max)=P(dissipated)*(R(chip)+R(from chip to heatsink)+R(heatsink))+T(room)
so from the datasheet i get it like this
70=35(1.5+0.25+X)+25 (the room temperature is about 25, and with termopaste R(from chip to heatsink) is about 0.25)
so i will decrease the max temperature by 15 (just for safety)
55=35(1.5+0.25+X)+25 --> 30/35=1.75+x
0.86=1.75+X --> X=-0.89 ... my question is... why is it negative... the rezistance cant be negative...

You need to fully understand the problem and its one whole big subject in itself.
The main points are that the connection between the two reservoir caps is not to be used as a ground point itself. You take a small (any length) spur off that connection and the end of that becomes our reference point or audio ground. Its electrically clean and has no current from the PSU lowing in it. This point is used as a star that the other signal grounds return too.

Heatsinks... I've never done thermal calculations and always choose empirically with an eye on the degrees C/per watt rating Again this is where the difference between pro and domestic applications come to the surface. The TDA7293 type chips might be rated at 100 watts RMS but in practice you are never going to get that heat away from the chip fast enough. So you compromise and choose something thats acceptable for your use.
Something like this,